Java常用分析工具之jmap

港控/mmm° 2022-05-24 08:50 347阅读 0赞

## 简介 ##

jmap是JDK中提供的一个用来监视进程运行中的jvm物理内存的占用情况的工具。该进程内存内，所有对象的情况，例如产生了哪些对象，对象数量。当系统崩溃时，jmap 可以从core文件或进程中获得内存的具体匹配情况，包括Heap size, Perm size等。

使用jmap会影响线上运行的应用，所以尽量不要在线上执行此命令。如果想dump堆信息，可以使用gcore命令，比jmap -dump快。

## 使用 ##

jmap的使用很简单，可以通过以下命令查看。

jmap -h
    Usage:
        jmap [option] <pid>
            (to connect to running process)
        jmap [option] <executable <core>
            (to connect to a core file)
        jmap [option] [server_id@]<remote server IP or hostname>
            (to connect to remote debug server)
    .......

## 案例 ##

### 查看堆heap占用情况 ###

jmap -heap 90099
    Attaching to process ID 90099, please wait...
    Debugger attached successfully.
    Server compiler detected.
    JVM version is 24.71-b01
    
    using thread-local object allocation.
    Parallel GC with 4 thread(s)  #垃圾回收的方式
    
    Heap Configuration: #堆配置信息
       MinHeapFreeRatio = 0
       MaxHeapFreeRatio = 100
       MaxHeapSize      = 2147483648 (2048.0MB)
       NewSize          = 1310720 (1.25MB)
       MaxNewSize       = 17592186044415 MB
       OldSize          = 5439488 (5.1875MB)
       NewRatio         = 2
       SurvivorRatio    = 8
       PermSize         = 21757952 (20.75MB)
       MaxPermSize      = 85983232 (82.0MB)
       G1HeapRegionSize = 0 (0.0MB)
    
    Heap Usage:
    PS Young Generation
    Eden Space:
       capacity = 34603008 (33.0MB)
       used     = 9773032 (9.320289611816406MB)
       free     = 24829976 (23.679710388183594MB)
       28.24330185398911% used
    From Space:
       capacity = 5242880 (5.0MB)
       used     = 0 (0.0MB)
       free     = 5242880 (5.0MB)
       0.0% used
    To Space:
       capacity = 5242880 (5.0MB)
       used     = 0 (0.0MB)
       free     = 5242880 (5.0MB)
       0.0% used
    PS Old Generation
       capacity = 89128960 (85.0MB)
       used     = 0 (0.0MB)
       free     = 89128960 (85.0MB)
       0.0% used
    PS Perm Generation
       capacity = 22020096 (21.0MB)
       used     = 2666808 (2.5432662963867188MB)
       free     = 19353288 (18.45673370361328MB)
       12.110791887555804% used
    
    677 interned Strings occupying 44048 bytes.

### 查看对象占用内存情况 ###

$ jmap -histo:live 96176
    
     num     #instances         #bytes  class name
    ----------------------------------------------
       1:          5948         766656  <methodKlass>
       2:          5948         679168  <constMethodKlass>
       3:           401         474448  <constantPoolKlass>
       4:           365         289664  <constantPoolCacheKlass>
       5:           401         276112  <instanceKlassKlass>
       6:           562          93176  [B
       7:           900          86256  [C
       8:           463          45328  java.lang.Class
       9:           653          43216  [[I
      10:           608          37144  [S
      11:            45          24480  <objArrayKlassKlass>

输出内容的一些说明：

\[C is a char\[\]

\[S is a short\[\]

\[I is a int\[\]

\[B is a byte\[\]

\[\[I is a int\[\]\[\]

上面的输出中\[C对象占用Heap这么多，往往跟String有关，String其内部使用final char\[\]数组来保存数据的。

constMethodKlass/ methodKlass/ constantPoolKlass/ constantPoolCacheKlass/ instanceKlassKlass/ methodDataKlass

与Classloader相关，常驻与Perm区。

### 导出堆信息 ###

可以使用jmap把堆信息导出，并使用可视化工具mat分析。导出的命令如下：

$ jmap -dump:format=b,file=a.dmp 96185
    Dumping heap to /Users/canglong/dev/test/a.dmp ...
    Heap dump file created